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Abstract:

Use of two different methods, either each by itself or in combination, to
enhance the stiffness, strength, maximum possible use temperature, and
environmental resistance of thermoset polymer particles is disclosed. One
method is the application of post-polymerization process steps (and
especially heat treatment) to advance the curing reaction and to thus
obtain a more densely crosslinked polymer network. The other method is
the incorporation of nanofillers, resulting in a heterogeneous
"nanocomposite" morphology. Nanofiller incorporation and
post-polymerization heat treatment can also be combined to obtain the
benefits of both methods simultaneously. The present invention relates to
the development of thermoset nanocomposite particles. Optional further
improvement of the heat resistance and environmental resistance of said
particles via post-polymerization heat treatment; processes for the
manufacture of said particles; and use of said particles in the
construction, drilling, completion and/or fracture stimulation of oil and
natural gas wells are described.

Claims:

1.-89. (canceled)

90. A method for fracture stimulation of a subterranean formation having
a wellbore, comprising: injecting into the wellbore a slurry containing a
nanocomposite spherical bead proppant at sufficiently high rates and
pressures such that said formation fails and fractures to accept said
slurry, said spherical bead proppant comprising: a polymer matrix; and
from 0.001 to 60 volume percent of nanofiller particles possessing a
length that is less than 0.5 microns in at least one principal axis
direction, wherein said nanofiller particles comprise at least one of
fine particulate material, fibrous material, discoidal material, or a
combination of such materials; and wherein said nanofiller particles are
selected from the group of nanofillers consisting of: natural nanoclays,
synthetic nanoclays or mixtures thereof, said nanofiller particles being
dispersed throughout said polymeric nanocomposite spherical bead
proppant, wherein said spherical bead proppant has a diameter ranging
from 0.1 mm to 4 mm and wherein said spherical bead proppant has about
neutral buoyancy relative to water; and emplacing said nanocomposite
spherical bead proppant within a fracture network in said formation in a
packed mass or a partial monolayer of said spherical head proppant, which
packed mass or partial monolayer props open the fracture network; thereby
allowing produced gases, fluids, or mixtures thereof, to flow towards the
wellbore.

91. The method of claim 90, wherein said polymer matrix comprises a
styrene-divinylbenzene copolymer or a
styrene-ethylvinylbenzene-divinylbenzene terpolymer.

92. The method of claim 91, wherein said polymer matrix contains
divinylbenzene in an amount ranging from 3% to 35% by weight.

93. The method of claim 90, wherein said nanofiller comprises natural
nanoclays, possessing a length that ranges between 100 and 500 nanometers
in at least one principal axis direction and present in an amount from
0.1% to 15% of said nanocomposite spherical bead proppant by volume.

94. The method of claim 90, wherein said nanofiller comprises synthetic
nanoclays, possessing a length that ranges between 100 and 500 nanometers
in at least one principal axis direction and present in an amount from
0.1% to 15% of said nanocomposite spherical bead proppant by volume.

95. The method of claim 90, wherein said packed mass or said partial
monolayer exhibits a static conductivity of at least 100 mDft after 200
hours at a temperature greater than 80.degree. F.

Description:

[0002] The present invention relates to lightweight thermoset polymer
nanocomposite particles, to processes for the manufacture of such
particles, and to applications of such particles. The particles of the
invention contain one or optionally more than one type of nanofiller that
is intimately embedded in the polymer matrix. It is possible to use a
wide range of thermoset polymers and nanofillers as the main constituents
of the particles of the invention, and to produce said particles by means
of a wide range of fabrication techniques. Without reducing the
generality of the invention, in its currently preferred embodiments, the
thermoset matrix consists of a terpolymer of styrene, ethylvinylbenzene
and divinylbenzene; particulate carbon black of nanoscale dimensions is
used as the nanofiller, suspension polymerization is performed in the
presence of the nanofiller, and optionally post-polymerization heat
treatment is performed with the particles still in the reactor fluid that
remains after the suspension polymerization to further advance the curing
of the matrix polymer. When executed in the manner taught by this patent,
many properties of both the individual particles and packings of said
particles can be improved by the practice of the invention. The particles
exhibit enhanced stiffness, strength, heat resistance, and resistance to
aggressive environments; as well as the improved retention of high
conductivity of liquids and gases through packings of said particles in
aggressive environments under high compressive loads at elevated
temperatures. The thermoset polymer nanocomposite particles of the
invention can be used in many applications. These applications include,
but are not limited to, the construction, drilling, completion and/or
fracture stimulation of oil and natural gas wells; for example, as a
proppant partial monolayer, a proppant pack, an integral component of a
gravel pack completion, a ball bearing, a solid lubricant, a drilling mud
constituent, and/or a cement additive.

BACKGROUND

[0003] The background of the invention can be described most clearly, and
hence the invention can be taught most effectively, by subdividing this
section in three subsections. The first subsection will provide some
general background regarding the role of crosslinked (and especially
stiff and strong thermoset) particles in the field of the invention. The
second subsection will describe the prior art that has been taught in the
patent literature. The third subsection will provide additional relevant
background information selected from the vast scientific literature on
polymer and composite materials science and chemistry, to further
facilitate the teaching of the invention.

A. General Background

[0004] Crosslinked polymer (and especially stiff and strong thermoset)
particles are used in many applications requiring high stiffness, high
mechanical strength, high temperature resistance, and/or high resistance
to aggressive environments. Crosslinked polymer particles can be prepared
by reacting monomers or oligomers possessing three or more reactive
chemical functionalities, as well as by reacting mixtures of monomers
and/or oligomers at least one ingredient of which possesses three or more
reactive chemical functionalities.

[0005] The intrinsic advantages of crosslinked polymer particles over
polymer particles lacking a network consisting of covalent chemical bonds
in such applications become especially obvious if an acceptable level of
performance must be maintained for a prolonged period (such as many
years, or in some applications even several decades) under the combined
effects of mechanical deformation, heat, and/or severe environmental
insults. For example, many high-performance thermoplastic polymers, which
have excellent mechanical properties and which are hence used
successfully under a variety of conditions, are unsuitable for
applications where they must maintain their good mechanical properties
for many years in the presence of heat and/or chemicals, because they
consist of assemblies of individual polymer chains. Over time, the
deformation of such assemblies of individual polymer chains at an
elevated temperature can cause unacceptable amounts of creep, and
furthermore solvents and/or aggressive chemicals present in the
environment can gradually diffuse into them and degrade their performance
severely (and in some cases even dissolve them). By contrast, the
presence of a well-formed continuous network of covalent bonds restrains
the molecules, thus helping retain an acceptable level of performance
under severe use conditions over a much longer time period.

[0006] Oil and natural gas well construction activities, including
drilling, completion and stimulation applications (such as proppants,
gravel pack components, ball bearings, solid lubricants, drilling mud
constituents, and/or cement additives), require the use of particulate
materials, in most instances preferably of as nearly spherical a shape as
possible. These (preferably substantially spherical) particles must
generally be made from materials that have excellent mechanical
properties. The mechanical properties of greatest interest in most such
applications are stiffness (resistance to deformation) and strength under
compressive loads, combined with sufficient "toughness" to avoid the
brittle fracture of the particles into small pieces commonly known as
"fines". In addition, the particles must have excellent heat resistance
in order to be able to withstand the combination of high compressive load
and high temperature that normally becomes increasingly more severe as
one drills deeper. In other words, particles that are intended for use
deeper in a well must be able to withstand not only the higher overburden
load resulting from the greater depth, but also the higher temperature
that accompanies that higher overburden load as a result of the nature of
geothermal gradients. Finally, these materials must be able to withstand
the effects of the severe environmental insults (resulting from the
presence of a variety of hydrocarbon and possibly solvent molecules as
well as water, at simultaneously elevated temperatures and compressive
loads) that the particles will encounter deep in an oil or natural gas
well. The need for relatively lightweight high performance materials for
use in these particulate components in applications related to the
construction, drilling, completion and/or fracture stimulation of oil and
natural gas wells thus becomes obvious. Consequently, while such uses
constitute only a small fraction of the applications of stiff and strong
materials, they provide fertile territory for the development of new or
improved materials and manufacturing processes for the fabrication of
such materials.

[0007] We will focus much of the remaining discussion of the background of
the invention on the use of particulate materials as proppants. One key
measure of end use performance of proppants is the retention of high
conductivity of liquids and gases through packings of the particles in
aggressive environments under high compressive loads at elevated
temperatures.

[0008] The use of stiff and strong solid proppants has a long history in
the oil and natural gas industry. Throughout most of this history,
particles made from polymeric materials (including crosslinked polymers)
have been considered to be unsuitable for use by themselves as proppants.
The reason for this prejudice is the perception that polymers are too
deformable, as well as lacking in the ability to withstand the
combination of elevated compressive loads, temperatures and aggressive
environments that are commonly encountered in oil and natural gas wells.
Consequently, work on proppant material development has focused mainly on
sands, on ceramics, and on sands and ceramics coated by crosslinked
polymers to improve some aspects of their performance. This situation has
prevailed despite the fact that most polymers have densities that are
much closer to that of water so that in particulate form they can be
transported much more readily into a fracture by low-density fracturing
or carrier fluids such as unviscosified water.

[0009] Nonetheless, the obvious practical advantages [see a review by
Edgeman (2004)] of developing the ability to use lightweight particles
that possess almost neutral buoyancy relative to water have stimulated a
considerable amount of work over the years. However, as will be seen from
the review of the prior art provided below, progress in this field of
invention has been very slow as a result of the many technical challenges
that exist to the successful development of cost-effective lightweight
particles that possess sufficient stiffness, strength and heat
resistance.

B. Prior Art

[0010] The prior art can be described most clearly, and hence the
invention can be placed in the proper context most effectively, by
subdividing this section into four subsections. The first subsection will
describe prior art related to the development of "as-polymerized"
thermoset polymer particles. The second subsection will describe prior
art related to the development of thermoset polymer particles that are
subjected to post-polymerization heat treatment. The third subsection
will describe prior art related to the development of thermoset polymer
composite particles where the particles are reinforced by conventional
fillers. The fourth subsection will describe prior art related to the
development of ceramic nanocomposite particles where a ceramic matrix is
reinforced by nanofillers.

1. "As-Polymerized" Thermoset Polymer Particles

[0011] As discussed above, particles made from polymeric materials have
historically been considered to be unsuitable for use by themselves as
proppants. Consequently, their past uses in proppant materials have
focused mainly on their placement as coatings on sands and ceramics, in
order to improve some aspects of the performance of the sand and ceramic
proppants.

[0012] Significant progress was made in the use of crosslinked polymeric
particles themselves as constituents of proppant formulations in prior
art taught by Rickards, et al. (U.S. Pat. No. 6,059,034; U.S. Pat. No.
6,330,916). However, these inventors still did not consider or describe
the polymeric particles as proppants. Their invention only related to the
use of the polymer particles in blends with particles of more
conventional proppants such as sands or ceramics. They taught that the
sand or ceramic particles are the proppant particles, and that the
"deformable particulate material" consisting of polymer particles mainly
serves to improve the fracture conductivity, reduce the generation of
fines and/or reduce proppant flowback relative to the unblended sand or
ceramic proppants. Thus while their invention differs significantly from
the prior art in the sense that the polymer is used in particulate form
rather than being used as a coating, it shares with the prior art the
limitation that the polymer still serves merely as a modifier improving
the performance of a sand or ceramic proppant rather than being
considered for use as a proppant in its own right.

[0013] Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress towards the
development of lightweight proppants consisting of high-strength
crosslinked polymeric particles for use in hydraulic fracturing
applications. However, embodiments of this prior art, based on the use of
styrene-divinylbenzene (S-DVB) copolymer beads manufactured by using
conventional fabrication technology and purchased from a commercial
supplier, failed to provide an acceptable balance of performance and
price. They cost far more than the test standard (Jordan sand) while
being outperformed by Jordan sand in terms of the liquid conductivity and
liquid permeability characteristics of their packings measured according
to the industry-standard API RP 61 testing procedure. [This procedure is
described by the American Petroleum Institute in its publication titled
"Recommended Practices for Evaluating Short Term Proppant Pack
Conductivity" (first edition, Oct. 1, 1989).] The need to use a very
large amount of an expensive crosslinker (50 to 80% by weight of DVB) in
order to obtain reasonable performance (not too inferior to that of
Jordan Sand) was a key factor in the higher cost that accompanied the
lower performance.

[0014] The most advanced prior art in stiff and strong crosslinked polymer
particle technologies for use in applications in oil and natural gas
drilling was developed by Albright (U.S. Pat. No. 6,248,838) who taught
the concept of a "rigid chain entanglement crosslinked polymer". In
summary, the reactive formulation and the processing conditions were
modified to achieve "rapid rate polymerization". While not improving the
extent of covalent crosslinking relative to conventional isothermal
polymerization, rapid rate polymerization results in the "trapping" of an
unusually large number of physical entanglements in the polymer. These
additional entanglements can result in a major improvement of many
properties. For example, the liquid conductivities of packings of S-DVB
copolymer beads with wDVB=0.2 synthesized via rapid rate
polymerization are comparable to those that were found by Bienvenu (U.S.
Pat. No. 5,531,274) for packings of conventionally produced S-DVB beads
at the much higher DVB level of wDVB=0.5. Albright (U.S. Pat. No.
6,248,838) thus provided the key technical breakthrough that enabled the
development of the first generation of crosslinked polymer beads
possessing sufficiently attractive combinations of performance and price
characteristics to result in their commercial use in their own right as
solid polymeric proppants.

2. Heat-Treated Thermoset Polymer Particles

[0015] There is no prior art that relates to the development of
heat-treated thermoset polymer particles for use in oil and natural gas
well construction applications. One needs to look into another field of
technology to find prior art of some relevance. Nishimori, et. al.
(JP1992-22230) focused on the development of particles for use in liquid
crystal display panels. They taught the use of post-polymerization heat
treatment to increase the compressive elastic modulus of S-DVB particles
at room temperature. They only claimed compositions polymerized from
reactive monomer mixtures containing 20% or more by weight of DVB or
other crosslinkable monomer(s) prior to the heat treatment. They stated
explicitly that improvements obtained with lower weight fractions of the
crosslinkable monomer(s) were insufficient and that hence such
compositions were excluded from the scope of their patent.

3. Thermoset Polymer Composite Particles

[0016] This subsection will be easier to understand if it is further
subdivided into two subsections. As was discussed above, the prior art on
the use of polymers as components of proppant particles has focused
mainly on the development of thermoset polymer coatings for rigid
inorganic materials such as sand or ceramic particles. These types of
heterogeneous (composite) particles will be discussed in the first
subsection. Composite particles where the thermoset polymer plays a role
that goes beyond that of a coating will be discussed in the second
subsection.

[0017] a. Thermoset Polymers as Coatings

[0018] The prior art discussed in this subsection is mainly of interest
for historical reasons, as examples of the evolution of the use of
thermoset polymers as components in composite proppant particles.

[0019] Underdown, et al. (U.S. Pat. No. 4,443,347) and of Glaze, et al.
(U.S. Pat. No. 4,664,819) taught the coating of particles such as silica
sand or glass beads with a thermoset polymer (such as a
phenol-formaldehyde resin) that is cured fully (in their terminology,
"pre-cured") prior to the injection of a proppant charge consisting of
such particles into a well.

[0020] An interesting alternative coating technology was taught by Graham,
et al. (U.S. Pat. No. 4,585,064) who developed resin-coated particles
comprising a particulate substrate, a substantially cured inner resin
coating, and a heat-curable outer resin coating. According to their
teaching, the outer resin coating should cure, and should thus enable the
particles to form a coherent mass possessing the desired level of liquid
conductivity, under the temperatures and compressive loads found in
subterranean formations. However, it is not difficult to anticipate the
many technical difficulties that can arise in attempting to reduce such
an approach reliably and consistently to practice.

[0022] McDaniel, et al. (U.S. Pat. No. 6,632,527) describes composite
particles made of a binder and filler; for use in subterranean formations
(for example, as proppants and as gravel pack components), in water
filtration, and in artificial turf for sports fields. The filler consists
of finely divided mineral particles that can be of any available
composition. Fibers are also used in some embodiments as optional
fillers. The sizes of the filler particles are required to fall within
the range of 0.5 microns to 60 microns. The proportion of filler in the
composite particle is very large (60% to 90% by volume). The binder
formulation is required to include at least one member of the group
consisting of inorganic binder, epoxy resin, novolac resin, resole resin,
polyurethane resin, alkaline phenolic resole curable with ester, melamine
resin, urea-aldehyde resin, urea-phenol-aldehyde resin, furans, synthetic
rubber, and/or polyester resin. The final thermoset polymer composite
particles of the required size and shape are obtained by a succession of
process steps such as the mixing of a binder stream with a filler
particle stream, agglomerative granulation, and the curing of granulated
material streams.

4. Ceramic Nanocomposite Particles

[0023] Nguyen, et al. (U.S. 20050016726) taught the development of ceramic
nanocomposite particles comprising a base material (present at roughly
50% to 90% by weight) and at least one nanoparticle material (present at
roughly 0.1% to 30% by weight). Optionally, a polymeric binder, an
organosilane coupling agent, and/or hollow microspheres, can also be
included. The base material comprises clay, bauxite, alumina, silica, or
mixtures thereof. It is stated that a suitable method for forming the
composite particulates from the dry ingredients is to sinter by heating
at a temperature of between roughly 1000° C. and 2000° C.,
which is a ceramic fabrication process. Given the types of formulation
ingredients used as base materials by Nguyen, et al. (U.S. 20050016726),
and furthermore the fact that even if they were to incorporate a
polymeric binder in an embodiment of their invention said polymeric
binder would not retain its normal chemical composition and polymer chain
structure when a particulate is sintered by heating it at a temperature
of between 1000° C. and about 2000° C., their composite
particulates consist of the nanofiller(s) dispersed in a ceramic matrix.

C. Scientific Literature

[0024] The development of thermoset polymer nanocomposites requires the
consideration of a vast and multidisciplinary range of polymer and
composite materials science and chemistry challenges. It is essential to
convey these challenges in the context of the fundamental scientific
literature.

[0025] Bicerano (2002) provides a broad overview of polymer and composite
materials science that can be used as a general reference for most
aspects of the following discussion. Many additional references will also
be provided below, to other publications which treat specific issues in
greater detail than what could be accommodated in Bicerano (2002).

[0027] It is essential, first, to review some fundamental aspects of the
curing of crosslinked polymers, which are applicable to such polymers
regardless of their form (particulate, coating, or bulk).

[0028] The properties of crosslinked polymers prepared by standard
manufacturing processes are often limited by the fact that such processes
typically result in incomplete curing. For example, in an isothermal
polymerization process, as the glass transition temperature (Tg) of
the growing polymer network increases, it may reach the polymerization
temperature while the reaction is still in progress. If this happens,
then the molecular motions slow down significantly so that further curing
also slows down significantly. Incomplete curing yields a polymer network
that is less densely crosslinked than the theoretical limit expected from
the functionalities and relative amounts of the starting reactants. For
example, a mixture of monomers might contain 80% DVB by weight as a
crosslinker but the final extent of crosslinking that is attained may not
be much greater than what was attained with a much smaller percentage of
DVB. This situation results in lower stiffness, lower strength, lower
heat resistance, and lower environmental resistance than the thermoset is
capable of manifesting when it is fully cured and thus maximally
crosslinked.

[0029] When the results of the first scan and the second scan of S-DVB
beads containing various weight fractions of DVB (wDVB), obtained by
Differential Scanning calorimetry (DSC), as reported by Bicerano, et al.
(1996) (see FIG. 1) are compared, it becomes clear that the low
performance and high cost of the "as purchased" S-DVB beads utilized by
Bienvenu (U.S. Pat. No. 5,531,274) are related to incomplete curing. This
incomplete curing results in the ineffective utilization of DVB as a
crosslinker and thus in the incomplete development of the crosslinked
network. In summary, Bicerano, et al. (1996), showed that the Tg of
typical "as-polymerized" S-DVB copolymers, as measured by the first DSC
scan, increased only slowly with increasing wDVB, and furthermore
that the rate of further increase of Tg slowed down drastically for
wDVB>0.08. By contrast, in the second DSC scan (performed on
S-DVB specimens whose curing had been driven much closer to completion as
a result of the temperature ramp that had been applied during the first
scan), Tg grew much more rapidly with wDVB over the entire
range of up to wDVB=0.2458 that was studied. The more extensively
cured samples resulting from the thermal history imposed by the first DSC
scan can, thus, be considered to provide much closer approximations to
the ideal theoretical limit of a "fully cured" polymer network.

[0032] As was illustrated by Bicerano, et al. (1996) for S-DVB copolymers
with wDVB of up to 0.2458, enhancing the state of cure of a
thermoset polymer network can increase Tg very significantly
relative to the Tg of the "as-polymerized" material. In practice,
the heat distortion temperature (HDT) is used most often as a practical
indicator of the softening temperature of a polymer under load. As was
shown by Takemori (1979), a systematic understanding of the HDT is
possible through its direct correlation with the temperature dependences
of the tensile (or equivalently, compressive) and shear elastic moduli.
For amorphous polymers, the precipitous decrease of these elastic moduli
as Tg is approached from below renders the HDT well-defined,
reproducible, and predictable. HDT is thus closely related to (and
usually slightly lower than) Tg for amorphous polymers, so that it
can be increased significantly by increasing Tg significantly.

[0033] The HDT decreases gradually with increasing magnitude of the load
used in its measurement. For example, for general-purpose polystyrene
(which has Tg=100° C.), HDT=95° C. under a load of
0.46 MPa and HDT=85° C. under a load of 1.82 MPa are typical
values. However, the compressive loads deep in an oil well or natural gas
well are normally far higher than the standard loads (0.46 MPa and 1.82
MPa) used in measuring the HDT. Consequently, amorphous thermoset polymer
particles can be expected to begin to deform significantly at a lower
temperature than the HDT of the polymer measured under the standard high
load of 1.82 MPa. This deformation will cause a decrease in the
conductivities of liquids and gases through the propped fracture, and
hence in the loss of effectiveness as a proppant, at a somewhat lower
temperature than the HDT value of the polymer measured under the standard
load of 1.82 MPa.

[0034] b. Mechanical Properties

[0035] As was discussed earlier, Nishimori, et. al. (JP1992-22230) used
heat treatment to increase the compressive elastic modulus of their S-DVB
particles (intended for use in liquid crystal display panels)
significantly at room temperature (and hence far below Tg).
Deformability under a compressive load is inversely proportional to the
compressive elastic modulus. It is, therefore, important to consider
whether one may also anticipate major benefits from heat treatment in
terms of the reduction of the deformability of thermoset polymer
particles intended for oil and natural gas drilling applications, when
these particles are used in subterranean environments where the
temperature is far below the Tg of the particles. As explained
below, the enhancement of curing via post-polymerization heat treatment
is generally expected to have a smaller effect on the compressive elastic
modulus (and hence on the proppant performance) of thermoset polymer
particles when used in oil and natural gas drilling applications at
temperatures far below their Tg.

[0036] Nishimori, et. al. (JP1992-22230) used very large amounts of DVB
(wDVB>>0.2). By contrast, much smaller amounts of DVB
(wDVB≦0.2) must be used for economic reasons in the "lower
value" oil and natural gas drilling applications. The elastic moduli of a
polymer at temperatures far below Tg are determined primarily by
deformations that are of a rather local nature and hence on a short
length scale. Some enhancement of the crosslink density via further
curing (when the network junctions created by the crosslinks are far away
from each other to begin with) will hence not normally have nearly as
large an effect on the elastic moduli as when the network junctions are
very close to each other to begin with and then are brought even closer
by the enhancement of curing via heat treatment. Consequently, while the
compressive elastic modulus can be expected to increase significantly
upon heat treatment when wDVB is very large, any such effect will
normally be less pronounced at low values of wDVB. In summary, it
can thus generally be expected that the enhancement of the compressive
elastic modulus at temperatures far below Tg will probably be small
for the types of formulations that are most likely to be used in the
synthesis of thermoset polymer particles for oil and natural gas drilling
applications.

[0039] As was pointed out by Takemori (1979), the addition of rigid
fillers has a negligible effect on the HDT of amorphous polymers.
However, nanocomposite materials and technologies had not yet been
developed in 1979. It is, hence, important to consider, based on the data
that have been gathered and the insights that have been obtained more
recently, whether nanofillers may be expected to behave in a
qualitatively different manner because of their geometric
characteristics.

[0040] A review article by Aharoni (1998) considered this question and
showed that three criteria must be considered. Here are the most relevant
excerpts from his article: "When a combination of the following three
conditions is fulfilled, then the glass transition temperature . . . may
be increased relative to that of the same polymer in the absence of these
three conditions . . . . First, very large surface area of a rigid
heterogeneous material in close contact with the amorphous phase of the
polymer. Such large surface areas may be obtained by having a rigid
additive material extremely finely ground, preferably to nanometer length
scale. Second, strong attractive interactions should exist between the
heterogeneous surfaces and the polymer. In the absence of strong
attractive interactions with the heterogeneous rigid surfaces, the chain
segments in the boundary layer are capable of relaxing to a state
approximating the bulk polymer and the Tg will be identical or very
slightly higher than that of the pure bulk polymer. Third, measure of
motional cooperation must exist between interchain and intrachain
fragments. Unlike the effects of high modulus heterogeneous additives on
the averaged modulus of the system in which they are present, the
elevation of Tg of the polymer matrix was repeatedly shown to
require not only that the polymer itself will be a high molecular weight
substance, but that the additive will be finely comminuted to generate
very large polymer-heterophase interfacial surface area, and, especially
important, that strong attractive interactions will exist between the
polymer and the foreign additive. These interactions are generally of an
ionic, hydrogen bonding, or dipolar nature and, as a rule, require that
the foreign additive will have surface energy higher than or at least
equal to, but never lower than, that of the amorphous polymer in which it
is being incorporated."

[0041] Almost by definition, Aharoni's first condition will be satisfied
for any nanofiller that has been dispersed well in the polymer matrix.
Furthermore, since a thermoset polymer contains a covalently bonded
three-dimensional network structure, his third condition will also be
satisfied if any thermoset polymer is used as the matrix material.
However, in most systems, there will not be strong attractive
interactions "generally of an ionic, hydrogen bonding, or dipolar nature"
between the polymer and the nanofiller, so that the second criterion will
not be satisfied. It can, therefore, be concluded that, for most
combinations of polymer and nanofiller, Tg will not increase
significantly upon incorporation of the nanofiller so that the maximum
possible use temperature will not increase significantly either. There
will, however, be exceptions to this general rule. Combinations of
polymer and nanofiller that manifest strong attractive interactions can
be found, and for such combinations both Tg and the maximum possible
use temperature can increase significantly upon nanofiller incorporation.

[0042] b. Mechanical Properties

[0043] It is well-established that the incorporation of rigid fillers into
a polymer matrix can produce a composite material which has significantly
greater stiffness (elastic modulus) and strength (stress required to
induce failure) than the base polymer. It is also well-established that
rigid nanofillers can generally stiffen and strengthen a polymer matrix
more effectively than conventional rigid fillers of similar composition
since their geometries allow them to span (or "percolate through") a
polymer specimen at much lower volume fractions than conventional
fillers. This particular advantage of nanofillers over conventional
fillers is well-established and a major driving force for the vast
research and development effort worldwide to develop new nanocomposite
products.

[0044] FIG. 2 provides an idealized schematic illustration of the
effectiveness of nanofillers in terms of their ability to "percolate
through" a polymer specimen even when they are present at a low volume
fraction. It is important to emphasize that FIG. 2 is of a completely
generic nature. It is presented merely to facilitate the understanding of
nanofiller percolation, without implying that it provides an accurate
depiction of the expected behavior of any particular nanofiller in any
particular polymer matrix. In practice, the techniques of electron
microscopy are generally used to observe the morphologies of actual
embodiments of the nanocomposite concept. Specific examples of the
ability of nanofillers such as carbon black and fumed silica to
"percolate" at extremely low volume fractions when dispersed in polymers
are provided by Zhang, et al (2001). The vast literature and trends on
the dependences of percolation thresholds and packing fractions on
particle shape, aggregation, and other factors, are reviewed by Bicerano,
et al. (1999).

[0045] As has also been studied extensively [for example, see Okamoto, et
al. (1999)] but is less widely recognized by workers in the field, the
incorporation of rigid fillers of appropriate types and dimensions in the
right amount (often just a very small volume fraction) can toughen a
polymer in addition to stiffening it and strengthening it. "Toughening"
implies a reduction in the tendency to undergo brittle fracture. If and
when it is realized for proppant particles, it is an important additional
benefit since it reduces the risk of the generation of "fines" during
use.

[0047] It is important to also review the many serious technical
challenges that exist to the successful incorporation of nanoparticles in
thermoset polymers. Appreciation of these obstacles can help workers in
the field of the invention gain a better understanding of the invention.
There are three major types of potential obstacles. In general, each
potential obstacle will tend to become more serious with increasing
nanofiller volume fraction, so that it is usually easier to incorporate a
small volume fraction of a nanofiller into a polymer than it is to
incorporate a larger volume fraction. This subsection is subdivided
further into the following three subsections where each type of major
potential obstacle will be discussed in turn.

[0048] a. Difficulty of Dispersing Nanofiller

[0049] The most common difficulty that is encountered in preparing polymer
nanocomposites involves the need to disperse the nanofiller. The specific
details of the source and severity of the difficulty, and of the methods
that may help overcome the difficulty, differ between types of
nanofillers, polymers, and fabrication processes (for example, the "in
situ" synthesis of the polymer in an aqueous or organic medium containing
the nanofiller, versus the addition of the nanofiller into a molten
polymer). However, some important common aspects can be identified.

[0050] Most importantly, nanofiller particles of the same kind often have
strong attractive interactions with each other. As a result, they tend to
"clump together"; for example, preferably into agglomerates (if the
nanofiller is particulate), bundles (if the nanofiller is fibrous), or
stacks (if the nanofiller is discoidal). In most systems, their
attractive interactions with each other are stronger than their
interactions with the molecules constituting the dispersing medium, so
that their dispersion is thermodynamically disfavored and hence extremely
difficult.

[0051] Even in systems where the dispersion of the nanofillers is
thermodynamically favored, it is often still very difficult to achieve
because of the large kinetic barriers (activation energies) that must be
surmounted. Consequently, nanofillers are very rarely easy to disperse in
a polymer.

[0052] b. High Dispersion Viscosity

[0053] Another difficulty with the fabrication of nanocomposites is the
fact that, once the nanofiller is dispersed in the appropriate medium
(for example, an aqueous or organic medium containing the nanofiller for
the "in situ" synthesis of the polymer, or a molten polymer into which
nanofiller is added), the viscosity of the resulting dispersion may (and
often does) become very high. When this happens, it can impede the
successful execution of the fabrication process steps that must follow
the dispersion of the nanofiller to complete the preparation of the
nanocomposite.

[0054] Dispersion rheology is a vast area of both fundamental and applied
research. It dates back to the 19th century, so that there is a vast
collection of data and a good fundamental understanding of the factors
controlling the viscosities of dispersions. Nonetheless, it is still at
the frontiers of materials science, so that major new experimental and
theoretical progress is continuing to be made. In fact, the advent of
nanotechnology, and the frequent emergence of high dispersion viscosity
as an obstacle to the fabrication of polymer nanocomposites, have been
instrumental in advancing the state of the art in this field. Bicerano,
et al. (1999) have provided a comprehensive overview which can serve as a
resource for workers interested in learning more about this topic.

[0055] C. Interference with Polymerization and Network Formation

[0056] An additional potential difficulty may be encountered in systems
where chemical reactions are taking place in a medium containing a
nanofiller. This is the possibility that the nanofiller may have an
adverse effect on the chemical reactions. As can reasonably be expected,
any such adverse effects can be far more severe in systems where
polymerization and network formation take place simultaneously in the
presence of a nanofiller than they can in systems where preformed polymer
chains are crosslinked in the presence of a nanofiller. The preparation
of an S-DVB nanocomposite via suspension polymerization in a medium
containing a nanofiller is an example of a process where polymerization
and network formation both take place in the presence of a nanofiller. On
the other hand, the vulcanization of a nanofilled rubber is a process
where preformed polymer chains are crosslinked in the presence of a
nanofiller.

[0057] The combined consideration of the work of Lipatov, et al. (1966,
1968), Popov, et al. (1982), and Bryk, et al. (1985, 1986, 1988) helps in
providing a broad perspective into the nature of the difficulties that
may arise. To summarize, the presence of a filler with a high specific
surface area can disrupt both polymerization and network formation in a
process such as the suspension polymerization of an S-DVB copolymer
nanocomposite. These outcomes can arise from the combined effects of the
adsorption of initiators on the surfaces of the nanofiller particles and
the interactions of the growing polymer chains with the nanofiller
surfaces. Adsorption on the nanofiller surface can affect the rate of
thermal decomposition of the initiator. Interactions of the growing
polymer chains with the nanofiller surfaces can result both in the
reduction of the mobility of growing polymer chains and in their
breakage. Very strong attractions between the initiator and the
nanofiller surfaces (for example, the grafting of the initiators on the
nanofiller surfaces) can potentially augment all of these detrimental
effects.

[0058] Taguchi, et al. (1999) provided a fascinating example of how
drastically the formulation can affect the particle morphology. They
described the results obtained by adding hydrophilic fine powders [nickel
(Ni) of mean particle size 0.3 microns, indium oxide (In2O3) of
mean particle size 0.03 microns, and magnetite (Fe3O4) of mean
particle size 0.1, 0.3 or 0.9 microns] to the aqueous phase during the
suspension polymerization of S-DVB. These particles had such a strong
affinity to the aqueous phase that they did not even go inside the S-DVB
beads. Instead, they remained entirely outside the beads. Consequently,
the composite particles consisted of S-DVB beads whose surfaces were
uniformly covered by a coating of inorganic powder. Furthermore, these
S-DVB beads rapidly became smaller with increasing amount of powder at a
fixed powder particle diameter, as well as with decreasing powder
particle diameter (and hence increasing number concentration of powder
particles) at a given powder weight fraction.

SUMMARY OF THE INVENTION

[0059] The present invention involves a novel approach towards the
practical development of stiff, strong, tough, heat resistant, and
environmentally resistant ultralightweight particles, for use in the
construction, drilling, completion and/or fracture stimulation of oil and
natural gas wells.

[0060] The disclosure is summarized below in three key aspects: (A)
Compositions of Matter (thermoset nanocomposite particles that exhibit
improved properties compared with prior art), (B) Processes (methods for
manufacture of said compositions of matter), and (C) Applications
(utilization of said compositions of matter in the construction,
drilling, completion and/or fracture stimulation of oil and natural gas
wells).

[0061] The disclosure describes lightweight thermoset nanocomposite
particles whose properties are improved relative to prior art. The
particles targeted for development include, but are not limited to,
terpolymers of styrene, ethylvinylbenzene and divinylbenzene; reinforced
by particulate carbon black of nanoscale dimensions. The particles
exhibit any one or any combination of the following properties: enhanced
stiffness, strength, heat resistance, and/or resistance to aggressive
environments; and/or improved retention of high conductivity of liquids
and/or gases through packings of said particles when said packings are
placed in potentially aggressive environments under high compressive
loads at elevated temperatures.

[0062] The disclosure also describes processes that can be used to
manufacture said particles. The fabrication processes targeted for
development include, but are not limited to, suspension polymerization in
the presence of nanofiller, and optionally post-polymerization heat
treatment with said particles still in the reactor fluid that remains
after the suspension polymerization to further advance the curing of the
matrix polymer.

[0063] The disclosure finally describes the use of said particles in
practical applications. The targeted applications include, but are not
limited to, the construction, drilling, completion and/or fracture
stimulation of oil and natural gas wells; for example, as a proppant
partial monolayer, a proppant pack, an integral component of a gravel
pack completion, a ball bearing, a solid lubricant, a drilling mud
constituent, and/or a cement additive.

A. Compositions of Matter

[0064] The compositions of matter of the present invention are thermoset
polymer nanocomposite particles where one or optionally more than one
type of nanofiller is intimately embedded in a polymer matrix. Any
additional formulation component(s) familiar to those skilled in the art
can also be used during the preparation of said particles; such as
initiators, catalysts, inhibitors, dispersants, stabilizers, rheology
modifiers, buffers, antioxidants, defoamers, impact modifiers,
plasticizers, pigments, flame retardants, smoke retardants, or mixtures
thereof. Some of the said additional component(s) may also become either
partially or completely incorporated into said particles in some
embodiments of the invention. However, the two required major components
of said particles are a thermoset polymer matrix and at least one
nanofiller. Hence this subsection will be further subdivided into three
subsections. Its first subsection will teach the volume fraction of
nanofiller(s) that may be used in the particles of the invention. Its
second subsection will teach the types of thermoset polymers that may be
used as matrix materials. Its third subsection will teach the types of
nanofillers that may be incorporated.

1. Nanofiller Volume Fraction

[0065] By definition, a nanofiller possesses at least one principal axis
dimension whose length is less than 0.5 microns (500 nanometers). This
geometric attribute is what differentiates a nanofiller from a finely
divided conventional filler, such as the fillers taught by McDaniel, et
al. (U.S. Pat. No. 6,632,527) whose characteristic lengths ranged from
0.5 microns to 60 microns.

[0066] The dispersion of a nanofiller in a polymer is generally more
difficult than the dispersion of a conventional filler of similar
chemical composition in the same polymer. However, if dispersed properly
during composite particle fabrication, nanofillers can reinforce the
matrix polymer far more efficiently than conventional fillers.
Consequently, while 60% to 90% by volume of filler is claimed by
McDaniel, et al. (U.S. Pat. No. 6,632,527), only 0.001% to 60% by volume
of nanofiller is claimed in the present invention.

[0067] Without reducing the generality of the present invention, a
nanofiller volume fraction of 0.1% to 15% is used in its currently
preferred embodiments.

2. Matrix Polymers

[0068] Any rigid thermoset polymer may be used as the matrix polymer of
the present invention.

[0069] Rigid thermoset polymers are, in general, amorphous polymers where
covalent crosslinks provide a three-dimensional network. However, unlike
thermoset elastomers (often referred to as "rubbers") which also possess
a three-dimensional network of covalent crosslinks, the rigid thermosets
are, by definition, "stiff". In other words, they have high elastic
moduli at "room temperature" (25° C.), and often up to much higher
temperatures, because their combinations of chain segment stiffness and
crosslink density result in a high glass transition temperature.

[0070] Some examples of rigid thermoset polymers that can be used as
matrix materials of the invention will be provided below. It is to be
understood that these examples are being provided without reducing the
generality of the invention, merely to facilitate the teaching of the
invention.

[0071] Rigid thermoset polymers that are often used as matrix (often
referred to as "binder") materials in composites include, but are not
limited to, crosslinked epoxies, epoxy vinyl esters, polyesters,
phenolics, polyurethanes, and polyureas. Rigid thermoset polymers that
are used less often because of their high cost despite their exceptional
performance include, but are not limited to, crosslinked polyimides.
These various types of polymers can, in different embodiments of the
invention, be prepared by starting either from their monomers, or from
oligomers that are often referred to as "prepolymers", or from suitable
mixtures of monomers and oligomers.

[0072] Many additional types of rigid thermoset polymers can also be used
as matrix materials in composites, and are all within the scope of the
invention. Such polymers include, but are not limited to, various
families of crosslinked copolymers prepared most often by the
polymerization of vinylic monomers, of vinylidene monomers, or of
mixtures thereof.

[0073] The "vinyl fragment" is commonly defined as the CH2═CH--
fragment. So a "vinylic monomer" is a monomer of the general structure
CH2═CHR where R can be any one of a vast variety of molecular
fragments or atoms (other than hydrogen). When a vinylic monomer
CH2═CHR reacts, it is incorporated into the polymer as the
--CH2--CHR-- repeat unit. Among rigid thermosets built from vinylic
monomers, the crosslinked styrenics and crosslinked acrylics are
especially familiar to workers in the field. Some other familiar types of
vinylic monomers (among others) include the olefins, vinyl alcohols,
vinyl esters, and vinyl halides.

[0074] The "vinylidene fragment" is commonly defined as the
CH2═CR''-- fragment. So a "vinylidene monomer" is a monomer of
the general structure CH2═CR'R'' where R' and R'' can each be
any one of a vast variety of molecular fragments or atoms (other than
hydrogen). When a vinylidene monomer CH2═CR'R'' reacts, it is
incorporated into a polymer as the --CH2--CR'R''-- repeat unit.
Among rigid thermosets built from vinylidene polymers, the crosslinked
alkyl acrylics [such as crosslinked poly(methyl methacrylate)] are
especially familiar to workers in the field. However, vinylidene monomers
similar to each type of vinyl monomer (such as the styrenics, acrylates,
olefins, vinyl alcohols, vinyl esters and vinyl halides, among others)
can be prepared. One example of particular interest in the context of
styrenic monomers is α-methyl styrene, a vinylidene-type monomer
that differs from styrene (a vinyl-type monomer) by having a methyl
(--CH3) group serving as the R'' fragment replacing the hydrogen
atom attached to the α-carbon.

[0075] Thermosets based on vinylic monomers, on vinylidene monomers, or on
mixtures thereof, are typically prepared by the reaction of a mixture
containing one or more non-crosslinking (difunctional) monomer and one or
more crosslinking (three or higher functional) monomers. All variations
in the choices of the non-crosslinking monomer(s), the crosslinking
monomers(s), and their relative amounts [subject solely to the limitation
that the quantity of the crosslinking monomer(s) must not be less than 1%
by weight], are within the scope of the invention.

[0076] Without reducing the generality of the invention, in its currently
preferred embodiments, the thermoset matrix consists of a terpolymer of
styrene (non-crosslinking), ethylvinylbenzene (also non-crosslinking),
and divinylbenzene (crosslinking), with the weight fraction of
divinylbenzene ranging from 3% to 35% by weight of the starting monomer
mixture.

3. Nanofillers

[0077] By definition, a nanofiller possesses at least one principal axis
dimension whose length is less than 0.5 microns (500 nanometers). Some
nanofillers possess only one principal axis dimension whose length is
less than 0.5 microns. Other nanofillers possess two principal axis
dimensions whose lengths are less than 0.5 microns. Yet other nanofillers
possess all three principal axis dimensions whose lengths are less than
0.5 microns. Any reinforcing material possessing one nanoscale dimension,
two nanoscale dimensions, or three nanoscale dimensions, can be used as
the nanofiller in embodiments of the invention. Any mixture of two or
more different types of such reinforcing materials can also be used as
the nanofiller in embodiments of the invention.

[0078] Some examples of nanofillers that can be incorporated into the
nanocomposites of the invention will be provided below. It is to be
understood that these examples are being provided without reducing the
generality of the invention, merely to facilitate the teaching of the
invention.

[0079] Nanoscale carbon black, fumed silica and fumed alumina, such as
products of these types that are currently being manufactured by the
Cabot Corporation, consist of aggregates of small primary particles. See
FIG. 3 for a schematic illustration of such an aggregate, and of a larger
agglomerate. The aggregates may contain many very small primary
particles, often arranged in a "fractal" pattern, resulting in aggregate
principal axis dimensions that are also shorter than 0.5 microns. These
aggregates (and not the individual primary particles that constitute
them) are, in general, the smallest units of these nanofillers that are
dispersed in a polymer matrix under normal fabrication conditions. The
available grades of such nanofillers include variations in specific
surface area, extent of branching (structure) in the aggregates, and
chemical modifications intended to facilitate dispersion in different
types of media (such as aqueous or organic mixtures). Some product types
of such nanofillers are also provided in "fluffy" grades of lower bulk
density that are easier to disperse than the base grade but less
convenient to transport and store since the same weight of material
occupies more volume when it is in its fluffy form. Some products grades
of such nanofillers are also provided pre-dispersed in an aqueous medium.

[0080] Carbon nanotubes, carbon nanofibers, and cellulosic nanofibers
constitute three other classes of nanofillers. When separated from each
other by breaking up the bundles in which they are often found and then
dispersed well in a polymer, they serve as fibrous reinforcing agents. In
different products grades, they may have two principal axis dimensions in
the nanoscale range (below 500 nanometers), or they may have all three
principal axis dimensions in the nanoscale range (if they have been
prepared by a process that leads to the formation of shorter nanotubes or
nanofibers). Currently, carbon nanotubes constitute the most expensive
nanofillers of fibrous shape. Carbon nanotubes are available in
single-wall and multi-wall versions. The single-wall versions offer the
highest performance, but currently do so at a much higher cost than the
multi-wall versions. Nanotubes prepared from inorganic materials (such as
boron nitride) are also available.

[0081] Natural and synthetic nanoclays constitute another major class of
nanofiller. Nanocor and Southern Clay Products are the two leading
suppliers of nanoclays at this time. When "exfoliated" (separated from
each other by breaking up the stacks in which they are normally found)
and dispersed well in a polymer, the nanoclays serve as discoidal
(platelet-shaped) reinforcing agents. The thickness of an individual
platelet is around one nanometer (0.001 microns). The lengths in the
other two principal axis dimensions are much larger. They range between
100 and 500 nanometers in many product grades, thus resulting in a
platelet-shaped nanofiller that has three nanoscale dimensions. They
exceed 500 nanometers, and thus result in a nanofiller that has only one
nanoscale dimension, in some other grades.

[0082] Many additional types of nanofillers are also available; including,
but not limited to, very finely divided grades of fly ash, the polyhedral
oligomeric silsesquioxanes, and clusters of different types of metals,
metal alloys, and metal oxides. Since the development of nanofillers is
an area that is at the frontiers of materials research and development,
the future emergence of yet additional types of nanofillers that are not
currently known may also be readily anticipated.

[0083] Without reducing the generality of the invention, in its currently
preferred embodiments, nanoscale carbon black grades supplied by Cabot
Corporation are being used as the nanofiller.

B. Processes

[0084] In most cases, the incorporation of a nanofiller into the thermoset
polymer matrix will increase the compressive elastic modulus uniformly
throughout the entire use temperature range (albeit usually not by
exactly the same factor at each temperature), while not increasing
Tg significantly. The resulting nanocomposite particles will then
perform better as proppants over their entire use temperature range, but
without an increase in the maximum possible use temperature itself. On
the other hand, if a suitable post-polymerization process step is applied
to the nanocomposite particles, in many cases the curing reaction will be
driven further towards completion so that Tg (and hence also the
maximum possible use temperature) will increase along with the increase
induced by the nanofiller in the compressive elastic modulus.

[0085] Processes that may be used to enhance the degree of curing of a
thermoset polymer include, but are not limited to, heat treatment (which
may be combined with stirring and/or sonication to enhance its
effectiveness), electron beam irradiation, and ultraviolet irradiation.
We focused mainly on the use of heat treatment in order to increase the
Tg of the thermoset matrix polymer, to make it possible to use
nanofiller incorporation and post-polymerization heat treatment as
complementary methods, to improve the performance characteristics of the
particles even further by combining the anticipated main benefits of each
method. FIG. 4 provides an idealized schematic illustration of the
benefits of implementing these methods and concepts.

[0086] The processes that may be used for the fabrication of the thermoset
nanocomposite particles of the invention have at least one, and
optionally two, major step(s). The required step is the formation of said
particles by means of a process that allows the intimate embedment of the
nanofiller in the polymer matrix. The optional step is the use of an
appropriate postcuring method to advance the curing reaction of the
thermoset matrix and to thus obtain a polymer network that approaches the
"fully cured" limit. Consequently, this subsection will be further
subdivided into two subsections, dealing with polymerization and with
postcure respectively.

1. Polymerization and Network Formation in Presence of Nanofiller

[0087] Any method for the fabrication of thermoset composite particles
known to those skilled in the art may be used to prepare embodiments of
the thermoset nanocomposite particles of the invention. Without reducing
the generality of the invention, some such methods will be discussed
below to facilitate the teaching of the invention.

[0088] The most practical methods for the formation of composites
containing rigid thermoset matrix polymers involve the dispersion of the
filler in a liquid (aqueous or organic) medium followed by the "in situ"
formation of the crosslinked polymer network around the filler. This is
in contrast with the formation of thermoplastic composites where melt
blending can instead also be used to mix a filler with a fully formed
molten polymer. It is also in contrast with the vulcanization of a filled
rubber, where preformed polymer chains are crosslinked in the presence of
a filler.

[0089] The implementation of such methods in the preparation of thermoset
nanocomposite particles is usually more difficult to accomplish in
practice than their implementation in the preparation of composite
particles containing conventional fillers. As discussed earlier, common
challenges involve difficulties in dispersing the nanofiller, high
nanofiller dispersion viscosity, and possible interferences of the
nanofiller with polymerization and network formation. Nonetheless, these
challenges can all be surmounted by making judicious choices of the
formulation ingredients and their proportions, and then also determining
and using the optimum processing conditions.

[0090] McDaniel, et al. (U.S. Pat. No. 6,632,527) prepared polymer
composite particles with thermoset matrix formulations. Their
formulations were based on at least one member of the group consisting of
inorganic binder, epoxy resin, novolac resin, resole resin, polyurethane
resin, alkaline phenolic resole curable with ester, melamine resin,
urea-aldehyde resin, urea-phenol-aldehyde resin, furans, synthetic
rubber, and/or polyester resin. They taught the incorporation of
conventional filler particles, whose sizes ranged from 0.5 microns to 60
microns, at 60% to 90% by volume. Their fabrication processes differed in
details depending on the specific formulation, but in general included
steps involving the mixing of a binder stream with a filler particle
stream, agglomerative granulation, and the curing of a granulated
material stream to obtain thermoset composite particles of the required
size and shape. These processes can also be used to prepare the thermoset
nanocomposite particles of the present invention, where nanofillers
possessing at least one principal axis dimension shorter than 0.5 microns
are used at a volume fraction that does not exceed 60% and that is far
smaller than 60% in the currently preferred embodiments. The processes of
McDaniel, et al. (U.S. Pat. No. 6,632,527) are, hence, incorporated
herein by reference.

[0091] As was discussed earlier, many additional types of thermoset
polymers can also be used as the matrix materials in composites. Examples
include crosslinked polymers prepared from various styrenic, acrylic or
olefinic monomers (or mixtures thereof). It is more convenient to prepare
particles of such thermoset polymers (as well as of their composites and
nanocomposites) by using methods that can produce said particles directly
in the desired (usually substantially spherical) shape during
polymerization from the starting monomers. (While it is a goal of this
invention to create spherical particles, it is understood that it is
exceedingly difficult as well as unnecessary to obtain perfectly
spherical particles. Therefore, particles with minor deviations from a
perfectly spherical shape are considered perfectly spherical for the
purposes of this disclosure.) Suspension (droplet) polymerization is the
most powerful method available for accomplishing this objective. Two main
approaches exist to suspension polymerization. The first approach is
isothermal polymerization which is the conventional approach that has
been practiced for many decades. The second approach is "rapid rate
polymerization" as taught by Albright (U.S. Pat. No. 6,248,838) which is
incorporated herein by reference. Without reducing the generality of the
invention, suspension polymerization as performed via the rapid rate
polymerization approach taught by Albright (U.S. Pat. No. 6,248,838) is
used in the current preferred embodiments of the invention.

[0092] As was discussed earlier and illustrated in FIG. 1 with the data of
Bicerano, et al. (1996), typical processes for the synthesis of thermoset
polymers may result in the formation of incompletely cured networks, and
may hence produce thermosets with lower glass transition temperatures and
lower maximum use temperatures than is achievable with the chosen
formulation of reactants. Furthermore, difficulties related to incomplete
cure may sometimes be exacerbated in thermoset nanocomposites because of
the possibility of interference by the nanofiller in polymerization and
network formation. Consequently, the use of an optional
post-polymerization process step (or a sequence of such process steps) to
advance the curing of the thermoset matrix of a particle of the invention
is an aspect of the invention. Suitable methods include, but are not
limited to, heat treatment (also known as "annealing"), electron beam
irradiation, and ultraviolet irradiation.

[0093] Post-polymerization heat treatment is a very powerful method for
improving the properties and performance of S-DVB copolymers (as well as
of many other types of thermoset polymers) by helping the polymer network
approach its "full cure" limit. It is, in fact, the most easily
implementable method for advancing the state of cure of S-DVB copolymer
particles. However, it is important to recognize that another
post-polymerization method (such as electron beam irradiation or
ultraviolet irradiation) may be the most readily implementable one for
advancing the state of cure of some other type of thermoset polymer. The
use of any suitable method for advancing the curing of the thermoset
polymer that is being used as the matrix of a nanocomposite of the
present invention after polymerization is within the scope of the
invention.

[0094] Without reducing the generality of the invention, among the
suitable methods, heat treatment is used as the optional
post-polymerization method to enhance the curing of the thermoset polymer
matrix in the preferred embodiments of the invention. Any desired thermal
history can be optionally imposed; such as, but not limited to,
isothermal annealing at a fixed temperature; nonisothermal heat exposure
with either a continuous or a step function temperature ramp; or any
combination of continuous temperature ramps, step function temperature
ramps, and/or periods of isothermal annealing at fixed temperatures. In
practice, while there is great flexibility in the choice of a thermal
history, it must be selected carefully to drive the curing reaction to
the maximum final extent possible without inducing unacceptable levels of
thermal degradation.

[0095] Any significant increase in Tg by means of improved curing
will translate directly into an increase of comparable magnitude in the
practical softening temperature of the polymer particles under the
compressive load imposed by the subterranean environment. Consequently, a
significant increase of the maximum possible use temperature of the
thermoset polymer particles is the most common benefit of advancing the
extent of curing by heat treatment.

[0096] A practical concern during the imposition of optional heat
treatment is related to the amount of material that is being subjected to
heat treatment simultaneously. For example, very small amounts of
material can be heat treated uniformly and effectively in vacuum; or in
any inert (non-oxidizing) gaseous medium, such as, but not limited to, a
helium or nitrogen "blanket". However, heat transfer in a gaseous medium
is not nearly as effective as heat transfer in an appropriately selected
liquid medium. Consequently, during the optional heat treatment of large
quantities of the particles of the invention (such as, but not limited
to, the output of a run of a commercial-scale batch production reactor),
it is usually necessary to use a liquid medium, and furthermore also to
stir the particles vigorously to ensure that the heat treatment is
applied as uniformly as possible. Serious quality problems may arise if
heat treatment is not applied uniformly; for example, as a result of the
particles that were initially near the heat source being overexposed to
heat and thus damaged, while the particles that were initially far away
from the heat source are not exposed to sufficient heat and are thus not
sufficiently postcured.

[0097] If a gaseous or a liquid heat treatment medium is used, said medium
may contain, without limitation, one or a mixture of any number of types
of constituents of different molecular structure. However, in practice,
said medium must be selected carefully to ensure that its molecules will
not react with the crosslinked polymer particles to a sufficient extent
to cause significant oxidative and/or other types of chemical
degradation. In this context, it must also be kept in mind that many
types of molecules which do not react with a polymer at ambient
temperature may react strongly with said polymer at elevated
temperatures. The most relevant example in the present context is that
oxygen itself does not react with S-DVB copolymers at room temperature,
while it causes severe oxidative degradation of S-DVB copolymers at
elevated temperatures where there would not be much thermal degradation
in its absence.

[0098] Furthermore, in considering the choice of medium for heat
treatment, it is also important to keep in mind that organic molecules
can swell organic polymers, potentially causing "plasticization" and thus
resulting in undesirable reductions of Tg and of the maximum
possible use temperature. The magnitude of any such detrimental effect
increases with increasing similarity between the chemical structures of
the molecules in the heat treatment medium and of the polymer chains. For
example, a heat transfer fluid consisting of aromatic molecules will tend
to swell a styrene-divinylbenzene copolymer particle, as well as tending
to swell a nanocomposite particle containing such a copolymer as its
matrix. The magnitude of this detrimental effect will increase with
decreasing relative amount of the crosslinking monomer (divinylbenzene)
used in the formulation. For example, a styrene-divinylbenzene copolymer
prepared from a formulation containing only 3% by weight of
divinylbenzene will be far more susceptible to swelling in an aromatic
liquid than a copolymer prepared from a formulation containing 35%
divinylbenzene.

[0099] Various means known to those skilled in the art, including but not
limited to the stirring and/or the sonication of an assembly of particles
being subjected to heat treatment, may also be optionally used to enhance
further the effectiveness of the optional heat treatment. The rate of
thermal equilibration under a given thermal gradient, possibly combined
with the application of any such additional means, depends on many
factors. These factors include, but are not limited to, the amount of
polymer particles being heat treated simultaneously, the shapes and
certain key physical and transport properties of these particles, the
shape of the vessel being used for heat treatment, the medium being used
for heat treatment, whether external disturbances (such as stirring
and/or sonication) are being used to accelerate equilibration, and the
details of the heat exposure schedule. Simulations based on the solution
of the heat transfer equations may hence be used optionally to optimize
the heat treatment equipment and/or the heat exposure schedule.

[0100] Without reducing the generality of the invention, in its currently
preferred embodiments, the thermoset nanocomposite particles are left in
the reactor fluid that remains after suspension polymerization if
optional heat treatment is to be used. Said reactor fluid thus serves as
the heat treatment medium; and simulations based on the solution of the
heat transfer equations are used to optimize the heat exposure schedule.
This embodiment of the optional heat treatment works especially well
(without adverse effects such as degradation and/or swelling) in
enhancing the curing of the thermoset matrix polymer in the currently
preferred compositions of matter of the invention. Said preferred
compositions of matter consist of terpolymers of styrene,
ethylvinylbenzene and divinylbenzene. Since the reactor fluid that
remains after the completion of suspension polymerization is aqueous
while these terpolymers are very hydrophobic, the reactor fluid serves as
an excellent heat transfer medium which does not swell the particles. The
use of the reactor fluid as the medium for the optional heat treatment
also has the advantage of simplicity since the particles would have
needed to be removed from the reactor fluid and placed in another fluid
as an extra step before heat treatment if an alternative fluid had been
required.

[0101] It is, however, important to reemphasize the much broader scope of
the invention and the fact that the particular currently preferred
embodiments summarized above constitute just a few among the vast variety
of possible qualitatively different classes of embodiments. For example,
if a hydrophilic thermoset polymer particle were to be developed as an
alternative preferred embodiment of the invention in future work, it
would obviously not be possible to subject such an embodiment to heat
treatment in an aqueous slurry, and a hydrophobic heat transfer fluid
would work better for its optional heat treatment.

C. Applications

[0102] The obvious practical advantages [see a review by Edgeman (2004)]
of developing the ability to use lightweight particles that possess
almost neutral buoyancy relative to water have stimulated a considerable
amount of work over the years. However, progress in this field of
invention has been very slow as a result of the many technical challenges
that exist to the successful development of cost-effective lightweight
particles that possess sufficient stiffness, strength and heat
resistance. The present invention has resulted in the development of such
stiff, strong, tough, heat resistant, and environmentally resistant
ultralightweight particles; and also of cost-effective processes for the
fabrication of said particles. As a result, a broad range of potential
applications can be envisioned and are being pursued for the use of the
thermoset polymer nanocomposite particles of the invention in the
construction, drilling, completion and/or fracture stimulation of oil and
natural gas wells. Without reducing the generality of the invention, in
its currently preferred embodiments, the specific applications that are
already being evaluated are as a proppant partial monolayer, a proppant
pack, an integral component of a gravel pack completion, a ball bearing,
a solid lubricant, a drilling mud constituent, and/or a cement additive.

[0103] It is also important to note that the current selection of
preferred embodiments of the invention has resulted from our focus on
application opportunities in the construction, drilling, completion
and/or fracture stimulation of oil and natural gas wells. Many other
applications can also be envisioned for the compositions of matter that
fall within the scope of thermoset nanocomposite particles of the
invention. For example, one such application is described by Nishimori,
et. al. (JP1992-22230), who developed heat-treated S-DVB copolymer (but
not composite) particles prepared from formulations containing very high
DVB weight fractions for use in liquid crystal display panels.
Alternative embodiments of the thermoset copolymer nanocomposite
particles of the present invention, tailored towards the performance
needs of that application and benefiting from its less restrictive cost
limitations, could potentially also be used in liquid crystal display
panels. Considered from this perspective, it can be seen readily that the
potential applications of the particles of the invention extend far
beyond their uses by the oil and natural gas industry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0104] The accompanying drawings, which are included to provide further
understanding of the invention and are incorporated in and constitute a
part of this specification, illustrate embodiments of the invention and,
together with the description, serve to explain the principles of the
invention.

[0105]FIG. 1 shows the effects of advancing the curing reaction in a
series of isothermally polymerized styrene-divinylbenzene (S-DVB)
copolymers containing different DVB weight fractions via heat treatment.
The results of scans of S-DVB beads containing various weight fractions
of DVB (wDVB), obtained by Differential Scanning calorimetry (DSC),
and reported by Bicerano, et al. (1996), are compared. It is seen that
the Tg of typical "as-polymerized" S-DVB copolymers, as measured by
the first DSC scan, increased only slowly with increasing wDVB, and
furthermore that the rate of further increase of Tg slowed down
drastically for wDVB>0.08. By contrast, in the second DSC scan
(performed on S-DVB specimens whose curing had been driven much closer to
completion as a result of the temperature ramp that had been applied
during the first scan), Tg grew much more rapidly with wDVB
over the entire range of up to wDVB=0.2458 that was studied.

[0106] FIG. 2 provides an idealized, generic and schematic two-dimensional
illustration of how a very small volume fraction of a nanofiller may be
able to "span" and thus "bridge through" a vast amount of space, thus
potentially enhancing the load bearing ability of the matrix polymer
significantly at much smaller volume fractions than possible with
conventional fillers.

[0107]FIG. 3 illustrates the "aggregates" in which the "primary
particles" of nanofillers such as nanoscale carbon black, fumed silica
and fumed alumina commonly occur. Such aggregates may contain many very
small primary particles, often arranged in a "fractal" pattern, resulting
in aggregate principal axis dimensions that are also shorter than 0.5
microns. These aggregates (and not the individual primary particles that
constitute them) are, usually, the smallest units of such nanofillers
that are dispersed in a polymer matrix under normal fabrication
conditions, when the forces holding the aggregates together in the much
larger "agglomerates" are overcome successfully. This illustration was
reproduced from the product literature of Cabot Corporation.

[0108]FIG. 4 provides an idealized schematic illustration, in the context
of the resistance of thermoset polymer particles to compression as a
function of the temperature, of the most common benefits of using the
methods of the present invention. In most cases, the densification of the
crosslinked polymer network via post-polymerization heat treatment will
have the main benefit of increasing the softening (and hence also the
maximum possible use) temperature, along with improving the environmental
resistance. On the other hand, in most cases, nanofiller incorporation
will have the main benefits of increasing the stiffness and strength. The
use of nanofiller incorporation and post-polymerization heat treatment
together, as complementary methods, will thus often be able to provide
all (or at least most) of these benefits simultaneously.

[0109]FIG. 5 provides a process flow diagram depicting the preparation of
the example. It contains four major blocks; depicting the preparation of
the aqueous phase (Block A), the preparation of the organic phase (Block
B), the mixing of these two phases followed by suspension polymerization
(Block C), and the further process steps used after polymerization to
obtain the "as-polymerized" and "heat-treated" samples of particles
(Block D).

[0110]FIG. 6 shows the variation of the temperature with time during
polymerization.

[0111]FIG. 7 shows the results of the measurement of the glass transition
temperatures (Tg) of the three heat-treated thermoset nanocomposite
samples via differential scanning calorimetry (DSC). The samples have
identical compositions. They differ only as a result of the use of
different heat treatment conditions after polymerization. Tg was
defined as the temperature at which the curve showing the heat flow as a
function of the temperature goes through its inflection point.

[0112] FIG. 8 provides a schematic illustration of the configuration of
the conductivity cell.

[0113]FIG. 9 shows the measured liquid conductivity of a packing of
particles of 14/16 U.S. mesh size (diameters ranging from 1.19 mm to 1.41
mm) from Sample 40 m200C, at a coverage of 0.02 lb/ft2, under a
closure stress of 4000 psi at a temperature of 190° F., as a
function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0114] Because the invention will be understood better after further
discussion of its currently preferred embodiments, further discussion of
said embodiments will now be provided. It is understood that said
discussion is being provided without reducing the generality of the
invention, since persons skilled in the art can readily imagine many
additional embodiments that fall within the full scope of the invention
as taught in the SUMMARY OF THE INVENTION section.

A. Nature, Attributes and Applications of Currently Preferred Embodiments

[0115] The currently preferred embodiments of the invention are
lightweight thermoset nanocomposite particles possessing high stiffness,
strength, temperature resistance, and resistance to aggressive
environments. These attributes, occurring in combination, make said
particles especially suitable for use in many challenging applications in
the construction, drilling, completion and/or fracture stimulation of oil
and natural gas wells. Said applications include the use of said
particles as a proppant partial monolayer, a proppant pack, an integral
component of a gravel pack completion, a ball bearing, a solid lubricant,
a drilling mud constituent, and/or a cement additive.

B. Thermoset Polymer Matrix

[0116] 1. Constituents

[0117] The thermoset matrix in said particles consists of a terpolymer of
styrene (S, non-crosslinking), ethylinylbenzene (EVB, also
non-crosslinking), and divinylbenzene (DVB, crosslinking). The preference
for such a terpolymer instead of a copolymer of S and DVB is a result of
economic considerations. To summarize, DVB comes mixed with EVB in the
standard product grades of DVB, and the cost of DVB increases rapidly
with increasing purity in special grades of DVB. EVB is a
non-crosslinking (difunctional) styrenic monomer. Its incorporation into
the thermoset matrix does not result in any significant changes in the
properties of the thermoset matrix or of nanocomposites containing said
matrix, compared with the use of S as the sole non-crosslinking monomer.
Consequently, it is far more cost-effective to use a standard (rather
than purified) grade of DVB, thus resulting in a terpolymer where some of
the repeat units originate from EVB.

[0118] 2. Proportions

[0119] The amount of DVB in said terpolymer ranges from 3% to 35% by
weight of the starting mixture of the three reactive monomers (S, EVB and
DVB) because different applications require different maximum possible
use temperatures. Even when purchased in standard product grades where it
is mixed with a large weight fraction of EVB, DVB is more expensive than
S. It is, hence, useful to develop different product grades where the
maximum possible use temperature increases with increasing weight
fraction of DVB. Customers can then purchase the grades of said particles
that meet their specific application needs as cost-effectively as
possible.

C. Nanofiller

[0120] 1. Constituents

[0121] The Monarch® 280 product grade of nanoscale carbon black
supplied by Cabot Corporation is being used as the nanofiller in said
particles. The reason is that it has a relatively low specific surface
area, high structure, and a "fluffy" product form; rendering it
especially easy to disperse.

[0122] 2. Proportions

[0123] The use of too low a volume fraction of carbon black results in
ineffective reinforcement. The use of too high a volume fraction of
carbon black may result in difficulties in dispersing the nanofiller,
dispersion viscosities that are too high to allow further processing with
available equipment, and detrimental interference in polymerization and
network formation. The amount of carbon black ranges from 0.1% to 15% by
volume of said particles because different applications require different
levels of reinforcement. Carbon black is more expensive than the monomers
(S, EVB and DVB) currently being used in the synthesis of the thermoset
matrix. It is, therefore, useful to develop different product grades
where the extent of reinforcement increases with increasing volume
fraction of carbon black. Customers can then purchase the grades of said
particles that meet their specific application needs as cost-effectively
as possible.

D. Polymerization

[0124] Suspension polymerization is performed via rapid rate
polymerization, as taught by Albright (U.S. Pat. No. 6,248,838) which is
incorporated herein by reference, for the fabrication of said particles.
Rapid rate polymerization has the advantage, relative to conventional
isothermal polymerization, of producing more physical entanglements in
thermoset polymers (in addition to the covalent crosslinks). Suspension
polymerization involves the preparation of an the aqueous phase and an
organic phase prior to the commencement of the polymerization process.
The Monarch® 280 carbon black particles are dispersed in the organic
phase prior to polymerization. The most important additional formulation
component (besides the reactive monomers and the nanofiller particles)
that is used during polymerization is the initiator. The initiator may
consist of one type molecule or a mixture of two or more types of
molecules that have the ability to function as initiators. Additional
formulation components, such as catalysts, inhibitors, dispersants,
stabilizers, rheology modifiers, buffers, antioxidants, defoamers, impact
modifiers, plasticizers, pigments, flame retardants, smoke retardants, or
mixtures thereof, may also be used when needed. Some of the additional
formulation component(s) may become either partially or completely
incorporated into the particles in some embodiments of the invention.

E. Attainable Particle Sizes

[0125] Suspension polymerization produces substantially spherical polymer
particles. (While it is a goal of this invention to create spherical
particles, it is understood that it is exceedingly difficult as well as
unnecessary to obtain perfectly spherical particles. Therefore, particles
with minor deviations from a perfectly spherical shape are considered
perfectly spherical for the purposes of this disclosure.) Said particles
can be varied in size by means of a number of mechanical and/or chemical
methods that are well-known and well-practiced in the art of suspension
polymerization. Particle diameters attainable by such means range from
submicron values up to several millimeters. Hence said particles may be
selectively manufactured over the entire range of sizes that are of
present interest and/or that may be of future interest for applications
in the oil and natural gas industry.

F. Optional Further Selection of Particles by Size

[0126] Optionally, after the completion of suspension polymerization, said
particles can be separated into fractions having narrower diameter ranges
by means of methods (such as, but not limited to, sieving techniques)
that are well-known and well-practiced in the art of particle
separations. Said narrower diameter ranges include, but are not limited
to, nearly monodisperse distributions. Optionally, assemblies of
particles possessing bimodal or other types of special distributions, as
well as assemblies of particles whose diameter distributions follow
statistical distributions such as gaussian or log-normal, can also be
prepared.

[0127] The optional preparation of assemblies of particles having diameter
distributions of interest from any given "as polymerized" assembly of
particles can be performed before or after any optional heat treatment of
said particles. Without reducing the generality of the invention, in the
currently most preferred embodiments of the invention, any optional
preparation of assemblies of particles having diameter distributions of
interest from the product of a run of the pilot plant or production plant
reactor is performed after the completion of any optional heat treatment
of said particles.

[0128] The particle diameters of current practical interest for various
uses in the construction, drilling, completion and/or fracture
stimulation of oil and natural gas wells range from 0.1 to 4 millimeters.
The specific diameter distribution that would be most effective under
given circumstances depends on the details of the subterranean
environment in addition to depending on the type of application. The
diameter distribution that would be most effective under given
circumstances may be narrow or broad, monomodal or bimodal, and may also
have other special features (such as following a certain statistical
distribution function) depending on both the details of the subterranean
environment and the type of application.

G. Optional Heat Treatment

[0129] Said particles are left in the reactor fluid that remains after
suspension polymerization if optional heat treatment is to be used. Said
reactor fluid thus serves as the heat treatment medium. This approach
works especially well (without adverse effects such as degradation and/or
swelling) in enhancing the curing of said particles where the polymer
matrix consists of a terpolymer of S, EVB and DVB. Since the reactor
fluid that remains after the completion of suspension polymerization is
aqueous while these terpolymers are very hydrophobic, the reactor fluid
serves as an excellent heat transfer medium which does not swell the
particles. The use of the reactor fluid as the medium for the optional
heat treatment also has the advantage of simplicity since the particles
would have needed to be removed from the reactor fluid and placed in
another fluid as an extra step before heat treatment if an alternative
fluid had been required.

[0130] Detailed and realistic simulations based on the solution of the
heat transfer equations are often used optionally to optimize the heat
exposure schedule if optional heat treatment is to be used. It has been
found that such simulations become increasingly useful with increasing
quantity of particles that will be heat treated simultaneously. The
reason is the finite rate of heat transfer. Said finite rate results in
slower and more difficult equilibration with increasing quantity of
particles and hence makes it especially important to be able to predict
how to cure most of the particles further uniformly and sufficiently
without overexposing many of the particles to heat.

Example

[0131] The currently preferred embodiments of the invention will be
understood better in the context of a specific example. It is to be
understood that said example is being provided without reducing the
generality of the invention. Persons skilled in the art can readily
imagine many additional examples that fall within the scope of the
currently preferred embodiments as taught in the DETAILED DESCRIPTION OF
THE INVENTION section. Persons skilled in the art can, furthermore, also
readily imagine many alternative embodiments that fall within the full
scope of the invention as taught in the SUMMARY OF THE INVENTION section.

A. Summary

[0132] The thermoset matrix was prepared from a formulation containing 10%
DVB by weight of the starting monomer mixture. The DVB had been purchased
as a mixture where only 63% by weight consisted of DVB. The actual
polymerizable monomer mixture used in preparing the thermoset matrix
consisted of roughly 84.365% S, 5.635% EVB and 10% DVB by weight.

[0133] Carbon black (Monarch 280) was incorporated into the particles, at
0.5% by weight, via dispersion in the organic phase of the formulation
prior to polymerization. Since the specific gravity of carbon black is
roughly 1.8 while the specific gravity of the polymer is roughly 1.04,
the amount of carbon black incorporated into the particles was roughly
0.29% by volume.

[0134] Suspension polymerization was performed in a pilot plant reactor,
via rapid rate polymerization as taught by Albright (U.S. Pat. No.
6,248,838) which is incorporated herein by reference. In applying this
method, the "dual initiator" approach, wherein two initiators with
different thermal stabilities are used to help drive the reaction of DVB
further towards completion, was utilized.

[0135] The required tests only require a small quantity of particles. The
use of a liquid medium (such as the reactor fluid) is unnecessary for the
heat treatment of a small sample. Roughly 500 grams of particles were
hence removed from the slurry, washed, spread very thin on a tray,
heat-treated for ten minutes at 200° C. in an oven in an inert gas
environment, and submitted for testing.

[0136] The glass transition temperature of these "heat-treated" particles,
and the liquid conductivity of packings thereof, were then measured by
independent testing laboratories (Impact Analytical in Midland, Mich.,
and FracTech Laboratories in Surrey, United Kingdom, respectively).

[0137]FIG. 5 provides a process flow diagram depicting the preparation of
the example. It contains four major blocks; depicting the preparation of
the aqueous phase (Block A), the preparation of the organic phase (Block
B), the mixing of these two phases followed by suspension polymerization
(Block C), and the further process steps used after polymerization to
obtain the "as-polymerized" and "heat-treated" samples of particles
(Block D).

[0138] The following subsections will provide further details on the
formulation, preparation and testing of this working example, to enable
persons who are skilled in the art to reproduce the example.

B. Formulation

[0139] An aqueous phase and an organic phase must be prepared prior to
suspension polymerization. The aqueous phase and the organic phase, which
were prepared in separate beakers and then used in the suspension
polymerization of the particles of this example, are described below.

[0140] 1. Aqueous Phase

[0141] The aqueous phase used in the suspension polymerization of the
particles of this example, as well as the procedure used to prepare said
aqueous phase, are summarized in TABLE 1.

[0143] The organic phase used in the suspension polymerization of the
particles of this example, as well as the procedure used to prepare said
organic phase, are summarized in TABLE 2. Note that the nanofiller
(carbon black) was added to the organic phase in this particular example.

[0144] Once the formulation is prepared, its aqueous and organic phases
are mixed, polymerization is performed, and "as-polymerized" and
"heat-treated" particles are obtained, as described below.

[0145] 1. Mixing

[0146] The aqueous phase was added to the reactor at 65° C. The
organic phase was then introduced over roughly 5 minutes with agitation
at the rate of 90 rpm. The mixture was held at 65° C. with
stirring at the rate of 90 rpm for at least 15 minutes or until proper
dispersion had taken place as manifested by the equilibration of the
droplet size distribution.

[0147] 2. Polymerization

[0148] The temperature was ramped from 65° C. to 78° C. in
10 minutes. It was then further ramped from 78° C. to 90°
C. at the rate of 0.1° C. per minute in 120 minutes. It was then
held at 90° C. for 90 minutes to provide most of the conversion of
monomer to polymer, with benzoyl peroxide (half life of one hour at
92° C.) as the effective initiator. It was then further ramped to
115° C. in 30 minutes and held at 115° C. for 180 minutes
to advance the curing with TAEC (half life of one hour at 117° C.)
as the effective initiator. The particles were thus obtained in an
aqueous slurry. FIG. 6 shows the variation of the temperature with time
during polymerization.

[0149] 3. "As-Polymerized" Particles

[0150] The aqueous slurry was cooled to 40° C. It was then poured
onto a 60 mesh (250 micron) sieve to remove the aqueous reactor fluid as
well as any undesirable small particles that may have formed during
polymerization. The "as-polymerized" beads of larger than 250 micron
diameter obtained in this manner were then washed three times with warm
(40° C. to 50° C.) water

[0151] 4. "Heat-Treated" Particles

[0152] Three sets of "heat-treated" particles, which were imposed to
different thermal histories during the post-polymerization heat
treatment, were prepared from the "as-polymerized" particles. In
preparing each of these heat-treated samples, washed beads were removed
from the 60 mesh sieve, spread very thin on a tray, placed in an oven
under an inert gas (nitrogen) blanket, and subjected to the desired heat
exposure. Sample 10 m200C was prepared with isothermal annealing for 10
minutes at 200° C. Sample 40 m200C was prepared with isothermal
annealing for 40 minutes at 200° C. to explore the effects of
extending the duration of isothermal annealing at 200° C. Sample
10 m220C was prepared with isothermal annealing for 10 minutes at
220° C. to explore the effects of increasing the temperature at
which isothermal annealing is performed for a duration of 10 minutes. In
each case, the oven was heated to 100° C., the sample was placed
in the oven and covered with a nitrogen blanket; and the temperature was
then increased to its target value at a rate of 2° C. per minute,
held at the target temperature for the desired length of time, and
finally allowed to cool to room temperature by turning off the heat in
the oven. Some particles from each sample were sent to Impact Analytical
for the measurement of Tg via DSC.

[0153] Particles of 14/16 U.S. mesh size were isolated from Sample 40
m200C by some additional sieving. This is a very narrow size
distribution, with the particle diameters ranging from 1.19 mm to 1.41
mm. This nearly monodisperse assembly of particles was sent to FracTech
Laboratories for the measurement of the liquid conductivity of its
packings.

D. Reference Sample

[0154] A Reference Sample was also prepared, to provide a baseline against
which the data obtained for the particles of the invention can be
compared.

[0155] The formulation and the fabrication process conditions used in the
preparation of the Reference Sample differed from those used in the
preparation of the examples of the particles of the invention in two key
aspects. Firstly, carbon black was not used in the preparation of the
Reference Sample. Secondly, post-polymerization heat treatment was not
performed in the preparation of the Reference Sample. Consequently, while
the examples of the particles of the invention consisted of a
heat-treated and carbon black reinforced thermoset nanocomposite, the
particles of the Reference Sample consisted of an unfilled and
as-polymerized thermoset polymer that has the same composition as the
thermoset matrix of the particles of the invention.

[0156] Some particles from the Reference Sample were sent to Impact
Analytical for the measurement of Tg via DSC. In addition, particles
of 14/16 U.S. mesh size were isolated from the Reference Sample by
sieving and sent to FracTech Laboratories for the measurement of the
liquid conductivity of their packings

E. Differential Scanning calorimetry

[0157] DSC experiments (ASTM E1356-03) were carried out by using a TA
Instruments Q100 DSC with nitrogen flow of 50 mL/min through the sample
compartment. Roughly nine milligrams of each sample were weighed into an
aluminum sample pan, the lid was crimped onto the pan, and the sample was
then placed in the DSC instrument. The sample was then scanned from
5° C. to 225° C. at a rate of 10° C. per minute. The
instrument calibration was checked with NIST SRM 2232 indium. Data
analysis was performed by using the TA Universal Analysis V4.1 software.

[0158] DSC data for the heat-treated samples are shown in FIG. 7. Tg
was defined as the temperature at which the curve for the heat flow as a
function of the temperature went through its inflection point. The
results are summarized in TABLE 3. It is seen that the extent of polymer
curing in Sample 10 m220C is comparable to that in Sample 40 m200C, and
that the extent of polymer curing in both of these samples has advanced
significantly further than that in Sample 10 m200C whose Tg was only
slightly higher than that of the Reference Sample.

TABLE-US-00003
TABLE 3
Glass transitions temperatures (Tg) of the three heat-treated
samples and of the Reference Sample, in ° C. In addition
to being an "as-polymerized" (rather than a heat-treated)
sample, the Reference Sample also differs from the other three
samples since it is an unfilled sample while the other three
samples each contain 0.5% by weight carbon black.
ISOTHERMAL HEAT TREATMENT Tg
SAMPLE IN NITROGEN (° C.)
Reference Sample None 117.17
10m200C For 10 minutes at a temperature of 200° C. 122.18
10m220C For 10 minutes at a temperature of 220° C. 131.13
40m200C For 40 minutes at a temperature of 200° C. 131.41

F. Liquid Conductivity Measurement

[0159] A fracture conductivity cell allows a particle packing to be
subjected to desired combinations of compressive stress (simulating the
closure stress on a fracture in a downhole environment) and elevated
temperature over extended durations, while the flow of a fluid through
the packing is measured. The flow capacity can be determined from
differential pressure measurements. The experimental setup is illustrated
in FIG. 8.

[0160] Ohio sandstone, which has roughly a compressive elastic modulus of
4 Mpsi and a permeability of 0.1 mD, was used as a representative type of
outcrop rock. Wafers of thickness 9.5 mm were machined to 0.05 mm
precision and one rock was placed in the cell. The sample was split to
ensure that a representative sample is achieved in terms of its particle
size distribution and then weighed. The particles were placed in the cell
and leveled. The top rock was then inserted. Heated steel platens were
used to provide the correct temperature simulation for the test. A
thermocouple inserted in the middle port of the cell wall recorded the
temperature of the pack. A servo-controlled loading ram provided the
closure stress. The conductivity of deoxygenated silica-saturated 2%
potassium chloride (KCl) brine of pH 7 through the pack was measured.

[0161] The conductivity measurements were performed by using the following
procedure:

[0162] 1. A 70 mbar full range differential pressure
transducer was activated by closing the bypass valve and opening the low
pressure line valve.

[0163] 2. When the differential pressure appeared to
be stable, a tared volumetric cylinder was placed at the outlet and a
stopwatch was started.

[0164] 3. The output of the differential pressure
transducer was fed to a data logger 5-digit resolution multimeter which
logs the output every second during the measurement.

[0165] 4. Fluid was
collected for 5 to 10 minutes, after which time the flow rate was
determined by weighing the collected effluent. The mean value of the
differential pressure was retrieved from the multimeter together with the
peak high and low values. If the difference between the high and low
values was greater than the 5% of the mean, the data point was
disregarded.

[0166] 5. The temperature was recorded from the inline
thermocouple at the start and at the end of the flow test period. If the
temperature variation was greater than 0.5° C., the test was
disregarded. The viscosity of the fluid was obtained from the measured
temperature by using viscosity tables. No pressure correction is made for
brine at 100 psi. The density of brine at elevated temperature was
obtained from these tables.

[0167] 6. At least three permeability
determinations were made at each stage. The standard deviation of the
determined permeabilities was required to be less than 1% of the mean
value for the test sequence to be considered acceptable.

[0168] 7. At the
end of the permeability testing, the widths of each of the four corners
of the cell were determined to 0.01 mm resolution by using vernier
calipers. The test results are summarized in TABLE 4.

[0169] These results are shown in FIG. 9. They demonstrate clearly the
advantage of the particles of the invention in terms of the enhanced
retention of liquid conductivity under a compressive stress of 4000 psi
at a temperature of 190° F.